Related Art
The present invention relates to methods and apparatus for manufacturing saws.
Traditionally, the manufacture of saws, such as bandsaws and straight blades has involved either the punching of tooth gullets in the edge of a single steel strip, or the milling of a plurality of toothed gullets simultaneously in a stack of parallel strips which are clamped together. Those methods have several important disadvantages. Punching as well as milling result in a lack of symmetry between the entry side and the exit side of the milling or punching tooling depending on the chip formation. This leads to external flaws such as rounding and burrs, as well as internal flaws such as micro-cracks and grain deformation. Even after a subsequent deburring, most of the flaws remain which can cause crooked sawing and uneven wear. In the above-described methods, the cross-wise edge is formed where the moving edge of the tooling intersects a previously formed contour of the strip, making it difficult to avoid micro-cracks and break-outs to the edge, where they are most harmful.
Saws for wood, where the demand for edge sharpness is especially high, are for this reason commonly sharpened after punching by filing or grinding.
Gullets can also be ground, which produces much better edge quality with modern grinding techniques. Disclosed in German Patent 903,884 is an abrasive grinding wheel and a rotary shaft disposed parallel to the steel strip. The wheel oscillates in a circle perpendicular to the plane of the strip to grind one gullet at a time, the strip being fed forwardly between each grinding operation.
According to German Patent 38 04 247, a plurality of tooth gullets are ground or sharpened simultaneously in a strip by a stack of abrasive wheels on a common shaft moving linearly in the plane of the strip.
According to Swedish Patent 455,384, a plurality of teeth are ground simultaneously in a stack of strips clamped together, whereby the abrasive wheel is moved perpendicularly to the plane of the strip during grinding, and the strip stack is moved lengthwise only between the grinding operations.
According to German Patent 43 32 894, a plurality of teeth are milled simultaneously in a stack of a few strips clamped together, where the milling cutter is moved in a line in the plane of the strips during the milling operation, and the package is moved lengthwise both during and between the milling operations in a programmed sequence comprising movements both forwards and backwards. A corresponding method can also be used for grinding.
According to U.S. Pat. No. 2,662,424, a saw can be sharpened by a small diameter abrasive peg or rotary file in a spindle which is perpendicular to the plane of the strip and moves in a closed curve in a plane accurately following the shape of the tooth gullet by means of two cam discs. The saw is fed forwardly between sharpening operations.
Because of the many movements to be started and stopped, the methods mentioned above need much time and suffer from vibration problems. When a stack of strips is used in the process, a large number of strips in the stack is needed for efficient productivity, which makes it difficult to reach the desired precision in clamping. Repeated starting and stopping of the feed mechanism causes rapid wear and deterioration of precision manufacturing.
Methods wherein the strip moves continuously and the abrasive wheel rotates on a fixed shaft have been tried in order to increase productivity and eliminate vibrations. According to U.S. Pat. No. 1,372,738 teeth can be milled in a strip moving continuously in a curved path by a helical cutter. German Patent 30 22 292 discloses that teeth can be ground in a strip moving continuously in a straight path by means of a conically helical abrasive wheel. These methods reach high rates of productivity, but set severe limits for tooth angles and tooth shapes; for example, teeth with varying pitch cannot be produced. Helical cutters and wheels are considerably more expensive to manufacture and maintain than those with rotational symmetry.
None of the methods mentioned above can be used to produce saws with chamfered teeth, and such teeth must as a rule be ground one-by-one with a stepwise longitudinal motion of the strip occurring between grinding operations. Stacks of short saw blades for handsaws can also be ground with chamfered teeth if they are realigned in precision fixtures between each grinding operation, but this is subject to severe restrictions as to tooth shape.
It would be desirable to provide a method and a machine for grinding of saws and similar tools as continuous strips, where the strip is continuously moving and teeth with irregular pitch and chamfers can be produced.